The present invention relates to a liquid crystal display device and, in particular, to a liquid crystal display device and method of driving the display which reduces the unevenness of the display.
A known method for driving a matrix-type liquid crystal display device is the voltage averaging method. However, since the signal and scanning electrodes have a resistance greater than zero, the liquid crystal layer acts as a dielectric. Therefore the effective voltages applied to the display elements (dots), defined at the intersection of each scanning electrode with a signal electrode, change depending on the characters and image displayed. As a result, unevenness of the display (uneven linear contrast) occurs.
Another driving method, known as the line reverse driving method, has been proposed to overcome the uneven contrast associated with the voltage averaging method. Disclosed in Japanese Patent Laid-Open Publication Nos. 62-31825, 60-19195 and 60-19196, the line reverse driving method involves inverting the polarity of the voltage applied to the liquid crystal panel multiple times during one frame.
The above described line reverse driving method is effective for improving the evenness of display caused by the variation in the optical characteristics of the liquid crystal layer caused by variations in the frequency of the applied voltages, however the unevenness is not completely remedied.
One further method described in Japanese Patent Application No. 63-159914 proposed by the present inventor is a voltage correcting method. While this method reduces unevenness of the display, further experimentation has revealed that utilization of this method still results in unevenness of display as described below.
Experimentation reveals that various causes have been determined to explain the unevenness of the display remaining even after the application of these prior art liquid crystal display driving methods. These causes are as follows, referring to FIGS. 1-4 as examples. FIG. 1 shows the structure of the liquid crystal display 1. Scanning electrodes Y1 to Y6 are arranged on the substrate 2 and signal electrodes X1 to X6 are arranged on the substrate 3. The intersection of a scanning electrode and a signal electrode is defined as a display element (dot) on the matrix display. A voltage is applied to each signal electrode. A lighting voltage is applied to the signal electrode if the corresponding display element is to be in the "ON" position (indicated by cross hatching in the drawings) while a "non-lighting" voltage is applied if the corresponding display element is to be in the "OFF" position. A scanning (selective) voltage is sequentially applied to scanning electrodes Y1-Y6 and then to scanning electrodes Y6-Y1. This scanning voltage is shifted from the first scanning electrode to the next scanning electrode at a predetermined time so that only one line of data is active at one time. As the selective or scanning voltages are applied in a particular order to scanning electrodes Y1 through Y6, lighting or non-lighting voltages are applied simultaneously to signal electrodes X1 through X6. A display element becomes illuminated (darkened) if the corresponding scanning electrode is selected and a lighting voltage is impressed on the corresponding signal electrode. If a non-lighting voltage is impressed on the signal electrode, the intersection of the signal electrode and the selected scanning electrode is a unilluminated display element. The liquid crystal display provides a "positive display". In other words the display element becomes dark, and is therefore displayed, when the effective voltage applied to the display element increases above a threshold. In order to avoid the application of a direct current to the display panel, the polarity of the signal and scanning voltages is reversed every frame. A frame is defined as the period of time it takes for the scanning voltage to be applied to each of scanning electrodes Y1 through Y6. Referring to FIGS. 3 and 4, one frame is indicated by F1 and the next frame of reversed polarity is indicated by F2.
If the resistance of the scanning electrodes Y1 through Y6 were the ideal, zero, a low-pass filter would be formed by the condenser defined by each display element, utilizing the dielectric of the liquid crystal and the resistance of the signal electrodes. Referring specifically to FIG. 2, R represents the resistance of a signal electrode X1-X6 and C represents the condenser formed by the display element. The ground represents the resistance of the signal electrode as being zero. In FIG. 2, damping occurs when the voltage across the condenser changes from positive to negative and from negative to positive relative to the scanning electrode. When this change between positive to negative occurs frequently, the effective voltage between the signal electrode and the scanning electrode becomes smaller. Referring to FIG. 1 for example, larger damping occur when the display elements at signal electrode X2 are changed from illuminated (ON) to nonilluminated (OFF) to illuminated to nonilluminated to illuminated (every other display element ON) when the scanning electrodes are scanned from the upper side (Y1 to Y6), than in the case in which the display elements formed at the signal electrode X4 are changed from nonilluminated (OFF) to illuminated (ON) to illuminated to illuminated to illuminated to nonilluminated when the scanning electrodes are scanned from the upper side (Y1 to Y6).
FIGS. 3 (a)-(c) and 4 (a)-(c) illustrate this principle. During the first frame, period F1, the voltages V0, V4, V5 and V3 are the selected, non-selected, lighting and non-lighting voltages respectively. During the second frame, period F2, the voltages V5, V1, V0 and V2 are the selected, non-selected, lighting, and non-lighting voltages respectively. FIG. 3(a) shows the voltage waveform of the signal electrode X2 which corresponds to the scanning electrode Y4. FIG. 3(b) shows the voltage waveform of the scanning electrode Y4. FIG. 3(c) shows the difference between the voltages of the signal electrode X2 and the scanning electrode Y4. Likewise, FIG. 4(a) shows the voltage waveform of the signal electrode X4 corresponding to the scanning electrode Y4. FIG. 10(b) shows the voltage waveform of the scanning electrode Y4 and FIG. 4(c) depicts the difference between the voltage waveforms of signal electrode X4 and the scanning electrode Y4. The hatched portions show the effect of damping on the variation from the ideal waveform.
Comparing FIGS. 3(c) and 4(c) it is apparent that more damping occurs at signal electrode X2 than at signal electrode X4. Specifically, the display elements on signal electrode X2 in this example are brighter than the dots on signal electrode X4, leading to nonuniformness of the display.
The number of changes in voltage between each signal electrode and the scanning electrode can be made uniform to some extent by using the line reversing driving method. This method can therefore partially alleviate the damping as it attempts to fix the effective voltage across the display elements. However, damping still occurs because the effective voltage cannot be made completely uniform, but rather, depends on the image being displayed.
By this invention, applicant further reduces the unevenness effect produced by the damping.